The United States and some European countries have for years expressed concern that Iran's nuclear-power ambitions could be used as cover for a nuclear-weapons program.

Now, as Tehran prepares to bring its Bushehr nuclear power plant online, independent nuclear experts tell RFE/RL that the focus of concern has shifted to Iran's uranium-enrichment efforts -- including the installation of centrifuges at its nuclear facility at Natanz.

Concerns over the possible misuse of Bushehr's nuclear fuel have eased following an agreement between Iran and its Russian partners, combined with monitoring by the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA).

Iran argues that it has a right under the Nuclear Nonproliferation Treaty it has signed to enrich uranium for use at nuclear power plants like Bushehr. But last year, under an agreement aimed at placating international concerns, Russia supplied Iran with the material that will be injected into Bushehr's nuclear fuel rods during the months ahead. Under that deal, estimated to be worth about $1 billion, Iran also has agreed to return all of Bushehr's spent nuclear fuel to Russia when it is removed from the reactor.

IAEA monitors already have sealed the enriched uranium delivered from Russia to Bushehr. The IAEA inspectors will also monitor the removal of spent fuel from Bushehr to ensure that none is diverted to an enrichment facility like the one at Natanz, where it could be transformed into the more highly enriched material needed to produce a nuclear weapon.

Shannon Kile, a nuclear nonproliferation expert at the Stockholm International Peace Research Institute, says that there are still legitimate concerns in Washington about Iran's nuclear-fuel-cycle program. But Kile says that those concerns are a separate issue to what is happening at Bushehr.

Kile says that what is really the "main driver" of international concerns "is a suspicion or fear that Iran may have undeclared enrichment activities under way -- that they may have a centrifuge plan other than" the Natanz facility.

"I think Bushehr has been eclipsed now," Kile adds. "It's not considered to be an especially important issue in terms of Iran having a nuclear-weapons program, except to the extent that it helps Iran to create a cadre of nuclear engineers and technicians who, in theory, could be used for a military nuclear program."

Enrichment Scenarios

Vitaly Fedchenko, a Russian-trained nuclear engineer and a researcher at the Stockholm International Peace Research Institute, says that there are two ways Bushehr's nuclear fuel could be enriched to the higher levels needed for a nuclear weapon.

While the IAEA knows about the uranium-enrichment complex at Natanz, are there other facilities they don't know of?

"One thing is to take the fresh fuel, somehow extract low-enriched uranium which is inside of it -- it is uranium dioxide -- and then convert it into the form suitable for further enrichment, uranium hexafluoride, and then feed it into the centrifuges," Fedchenko says.

But Fedchenko says that while this is possible "in theory," "in practice, it is highly improbable because it is going to take a lot of time to do that. And it would definitely be known [by IAEA inspectors and the international community] well before [the process is completed]."

Fedchenko says a second way for Iran to transform Bushehr's fuel into weapons-grade material would be to extract plutonium from the reactor's spent fuel. But he says that also is an unlikely scenario.

"That plutonium would have something which is called 'high burnup.' Its isotopic composition would be very badly suited for nuclear weapons," he says. "It would be highly unreliable."

Again, Fedchenko notes, IAEA inspectors would be aware immediately of any spent fuel from Bushehr that was not sent back to Russia as promised under current safeguard agreements.

"My conclusion is that both scenarios are going to be detected by the IAEA very quickly, very easily, and don't really make much sense from the physics point of view," Fedchenko says.

"Admittedly, yes, it is possible in theory. But in practice, it would be much easier for Iran to just abandon the whole Nonproliferation Treaty altogether and go its separate way than to pursue something like I have described," he continues. "It is not very feasible from a physics point of view. And it would be plain stupid from the political point of view."

Kile agrees that it would be foolish for Tehran to withdraw from the Nuclear Nonproliferation Treaty (NPT), an international accord signed by Iran that is aimed at limiting the global spread of nuclear weapons.

"There have been some discussions inside of Iran -- some conservative members of parliament have suggested that Iran should, as a response to the sanctions regime, simply withdraw from the Nonproliferation Treaty," Kile says.

"I don't think any responsible policymaker in Iran would even consider that at the moment because it would be considered as a cause of war. I think you would see Israeli air strikes, possibly abetted by the United States, within a relatively short time after that."

Suspicious Activity

Iran says that its uranium-enrichment facilities are part of its civilian nuclear-energy program, which is allowed under the Nonproliferation Treaty. But the IAEA already has found Tehran in noncompliance with its NPT safeguard agreements -- a decision that has led to international sanctions on Iran.

The latest IAEA report suggested last week that Iran's nuclear activities have slowed down, but that Tehran already has built up a stockpile of nuclear fuel that might be enriched to weapons-grade material.

Kile says that is why international concerns now focus on the possibility that Iran may have a secret enrichment program. "Those who criticize Iran's nuclear program, or are concerned about it, believe that the scale of the enrichment program is not what is needed for what Iran has said it is going to produce in terms of nuclear power plants," he says. "Others argue that it doesn't make economic sense for Iran to embark on its own nuclear-fuel-cycle program.

"The suspicion, given the outstanding safeguards-compliance questions that the IAEA has raised with Iran, is that Iran's nuclear program, in fact, has a military dimension. And there are a number of activities that Iran has engaged in over the past 10 or 15 years which, frankly, are very difficult to explain in terms of having applications for a peaceful nuclear-energy program," Kile says.

Mark Fitzpatrick, a nonproliferation expert at the London-based International Institute for Strategic Studies, notes that IAEA Secretary-General Muhammad el-Baradei's mandate is set to expire this summer -- just about the time that Bushehr is expected to start producing electricity. Fitzpatrick says he has no doubt that Iran will be the foremost challenge for el-Baradei's successor.